Then I get to replace the standard WD 500GB SATA with a SanDisk SSD. Yet another problem. No matter which way I approach the issue - Legacy Mode, Secure Boot Mode, Intel controller drivers, reformatting with NTFS, trying exFAT - you name it, Windows 10 flatly refuses to touch the SSD. The same make, modle and capacity that we have currently installed in several other HP desktops - all running Windows 10 x64. Funnily enough - that's why we've continued to buy these items from SanDisk. I've since put that SSD into another PC and it works fine.
I put in a cleanly wiped SATA drive into the 400 G3 SFF and bingo - it has no issues installing Windows.
I just don't know what's happened with this line of PCs, but they are genuinely awful. It's not like they are inexpensive to buy either and it's made me extremely wary of going anywhere near other HP SFF PCs. We have nearly 500 installs across our network and never have I come across just one specific machine causing so much unnecessary annoyance.